The invention relates to a lens system, in particular a projection lens system in semiconductor lithography, of the type defined more specifically in the preamble of claim 1.
In U.S. Pat. No. 5,353,166 there is a description of a projection lens system which comprises a stack mount with very solid individual parts. The rigidity of the solid parts is intended to achieve the effect that the separating surface is not deformed. The separation and a reconnection are consequently highly reproducible. The connection surfaces are in this case precision-ground. The lens system is also provided with intermediate rings or adjusting rings.
With high-performance lens systems, as is the case for example with projection lens systems in semiconductor lithography, it must be possible for residual imaging errors following an adjustment of the lens system to be minimized.
However, there is the risk of the imaging quality of lens systems of this type being altered by mechanical influences of known mounting techniques when mounted lenses are later removed and fitted. This is caused, for example, by the adjusting rings becoming loose when an air clearance is opened, by intercalated Teflon grease, by flatness errors of mounts and adjusting rings and by different tightening torques at the screws by which the outer mounts are connected to one another.
This means that, during removal and fitting of the same lens, or of another lens, fitting errors may occur as new errors and invalidate the desired improvement, and in some circumstances even impair the quality. The present invention is therefore based on the object of providing a lens system of the type mentioned at the beginning in which no new errors occur after a lens has been exchanged and refitting carried out, or by means of which reproducible refitting is achieved.
This object is achieved according to the invention by the features stated in the defining clause of claim 1.
The three-point mounting according to the invention allows complete isolation in terms of deformation and precise mounting, so that, even in the case of removal and subsequent refitting, reproducible conditions and/or a precise fitting position of the newly fitted optical element, such as for example a lens, which is identical to the previous position, are/is achieved.
The three-point mounting may be formed by three bearing bodies which are arranged in the outer mount and are provided with spherical bearing surfaces which are mounted in or on bearing locations of the inner mount.
Suitable for this purpose are, for example, three bearing locations which are formed by a planar supporting location, a channel-shaped supporting location and a fixed-point support.
According to the invention, in this case the continuation of the longitudinal axis of the channel-shaped supporting location should run through the fixed-point support, which may be formed by a conical receiving part. In this way, a very precisely defined position is created.
High positional reproducibility can be achieved by the use of hard metal elements and a solution with balls situated below or in the outer mount. The three-point mounting is self-centering during removal/fitting on account of the dead weight of the mounted lens. This means that, for example, the newly fitted lens no longer has to be recentered as a correction element itself, which further improves the reproducibility and also simplifies the fitting work.
Lens systems in semiconductor lithography are generally not tilted and also do not fall over. For this reason, no special fixing would be required. However, for safety reasons, the inner mount should be provided with a securing device preventing significant displacement or falling out. However, it is important here that no forces are transferred to the optical element, for example the lens. This may be carried out for example by means of an air gap of several micrometers or by a layer of adhesive, which however must not exert any pressure.